Venous air embolism frequently occurs in patients undergoing neck and head surgery in the sitting position and occasionally in patients operated on in the lateral, prone, and supine (head-up) positions. If sufficient air enters the circulation, the consequence may be cardiovascular failure resulting in death. Early detection of small amounts of air allows preventive therapeutic measures. The precordial Doppler ultrasonic approach used presently for this purpose suffers from: difficulty in accurate transducer placement, uncertainty in accurate placement, maintenance of placement in the sitting and other positions, no record of air embolized, no quantification of the volume of air embolized, and intermittent failure in detection of emboli in obese patients and those with large chests or breasts. We plan to exploit the normally used intravascular catheter and/or esophageal catheter as a transducer site for use of Doppler ultrasound to overcome these difficulties. In addition, we plan to evaluate the ability of intravascular Doppler sound for placement of the right atrial catheter tip used to aspirate air. Intravascular and esophageal units will be tested in animals and then in humans. The advantages and disadvantages of these approaches will be documented and comparison will be made to the precordial approach.